More About Rock Wool

Insulation, just like so many LEED building options was a rather tortuous decision. I was introduced to the idea of using rock wool by Sarah, my LEED expert friend. She recommended looking at rock wool for a couple of reasons. It has a bit higher R value plus it can have a high recycled content. It is often used commercially because it has a higher fire resistance and much higher melting point (2150 degrees vs. 1300 for fiberglass).

The technical bulletin from the Roxul site states the following advantages for rock wool batts.

The material safety data sheet reveals that the product did cause cancer in rats injected with the substance, but not in rats subjected to inhalation. The product is classified as not cancer causing. However besides the main ingredient of mineral wool, urea formaldehyde is used as a binder.

Cured Urea Extended Phenolic
Formaldehyde Binder 1-6%

This amount is the initial proportion according to this Green Building article. The heat process during manufacturing eliminates most of the chemical so that the final concentration is less than 0.0135 ppm which qualifies for Greenguard Clean Air certification.

Most US manufacturers of fiberglass insulation have stopped using urea formaldehyde in their binders entirely. And the classification of fiberglass was changed in 2001 from possibly causing cancer to not cancer causing, although there are inhalation studies where rats had a higher incidence of cancer. The MSDS list ingredients that are considered cancer causing* but only in California. The Certainteed and Owens Corning residential fiberglass products are also Greenguard certified.

Many sources consider cellulose insulation the greenest option. I was not impressed with the lower R value, the moisture holding qualities or the rodent resistance of cellulose, despite being treated with repellant and fire resistance. The cost for installation was about the same as the raw material cost for the Roxul, for the lower R value. Although articles sometimes treat the insulation values of these all the same, their actual ratings seemed to vary enough to tip the balance in favor of Roxul for our application. Foam has a much higher R value per inch, but the foam installer only quoted for 6 inches or about R 40 in the ceiling and 3″ or R 20 in the walls for $10,000 more than the rock wool cost.

I was also interested in a material that would not harbor rodents or mold. Rock wool is purported to be resistant to both. But my research indicates that both fiberglass and rock wool can gather dust and fibrous material that will harbor mold. Rock wool is also used as a hydroponic grow medium specifically because it can hold moisture for plant roots, but fiberglass also can be used because it also holds water, however, I have not found a direct comparison of the moisture holding properties of fiberglass to rock wool.

Although rock wool is considered unattractive to rodents, according to Wickipedia as well as vendor sites, a study done at the University of Nebraska and presented at a conference in 1992, states that all insulation materials, including rigid foams, are susceptible to infestations and the resulting damage.

The energy cost of manufacturing and the use of recycled content is about the same for fiberglass and rock wool. Both use high heat in their manufacturing process. Roxul states that the recycled content is variable but they use at least 40% recycled materials and the new Owens Corning Ecotouch uses at least 50% .

The materials cost to us was between 40% to 55% more for the rock wool. We paid about $1.25 per square foot of 5 1/2″ rock wool and fiberglass would have cost between 57 and 75 cents a square foot for unfaced fiberglass. Although new fiberglass would have cost about $2000 less, I pulled enough mouse infested, moldy fiberglass out of the house to be sure I wanted to try something different.

Installation is supposed to be easier with rock wool batts, they are firm and do not sag during installation. A comparison to fiberglass is made on the Lamidesign blog that addresses the water holding, R-value, and fire resistance of Roxul. I was swayed by the arguments in favor of Roxul.

The MSDS recommends masks and covering clothes and heads during installation just like fiberglass insulation, but apparently the dust does not stick to skin and clothes as fiberglass does. We’ll have to do our best to be sure that air barrier sealing eliminates mouse infiltration as much as possible and guard against moisture infiltration in the future.